Selecting biological control agents

Selecting effective agents

Selecting more effective agents

Characteristics of successful biological control agents

Perhaps the most successful predictor of how successful an agent will be is
its successful use elsewhere. Julien et al. (1999) recorded many examples
of the transfer of successful control agents from place to place. The 'Silwood
Project' analysed the success or failure of all weed control projects and
attempted to draw conclusions about which attributes generally led to control.
Conclusions proved difficult, but Crawley (1989) stated that weevils
(Coleoptera: Curculionidae) and chrysomelids (Coleoptera: Chrysomelidae) had
greater average success rate than other orders. The project also found that
characteristics that predicted ability to establish also broadly predicted
success, including small size, high voltinism, high fecundity, and long-lived
adults. Plant characters that appeared to make plants more susceptible to
biological control included genetic uniformity, lack of perennating organs, and
susceptibility to secondary infections. Plant characteristics that militated
against success included possession of rhizomes, high powers of regrowth and low
food quality. Morin et al. (1997) evaluated the relative performance of three
agents for mistflower (Ageratina riparia) in Hawaii before selecting a
biological control strategy for New Zealand. Froud and Stevens (1998) and
Froud and Stevens (2004) selected Thripobius semiluteus as a control agent for
glasshouse thrips because it was known to have contributed to control of the
thrips in closed habitats in Australia and elsewhere.

The success of related natural enemies and species in the same guild may also
help predict success. Conversely, failure elsewhere should contraindicate use in
New Zealand, but only if the reason for failure is known. However, this has not
stopped the transmission of gorse spider mite worldwide, despite its
susceptibility to generalist predators in New Zealand, and wherever it has been
released.

Price (2000) contends that bottom-up regulation of insect herbivore
populations through plant quality is more common than top-down regulation by
natural enemies. Eruptive species tend to be those that feed on a range of
plants. This creates the paradox that selecting agents for a high degree of
host-specificity also reduces the chance of selecting agents that can outbreak.
Stability of the parasitoid-host relationship sometimes reduces control
potential.

Mills and Gutierrez (1999) stated that understanding what leads to lasting
depression in pest population requires consideration of three processes: spatial
heterogeneity, parasitoid coexistence, and tritrophic interactions. Other papers
in Hawkins and Cornell (1999) examine the theory surrounding the roles played by
ecological factors in the development and implementation of biological control
of pests such as spatial heterogeneity and refuges, tritrophic relationships and
genetics. Thomas and Waage (1996) demonstrated convincingly how both bottom-up
and top-down effects may lead to better pest control than either alone.

McClay and Balciunas (2005) suggested that a pre-release efficacy assessment
(PREA) should be conducted as an additional filter in the agent selection
process. Although based on weed control agents, their principles of PREA apply
equally to arthropod agents. They suggested that predicted impact of an agent
could be estimated as:

Impact = Range × Abundance x Per-Capita Impact

Range is estimated by such factors as climatic limits, survival and
dispersal, geographic range, diapause and aestivation requirements, and climate
matching. Abundance is governed by such factors as voltinism, fecundity, host
suitability and survivorship. Per-capita effect can be estimated from native
field range studies, experimental manipulation, and from constructs like 'damage
curves' - change in fitness with agent load. Many of these contributing factors
are discussed in more detail below. In fact, the requirements of the HSNO Act
mean that an application to release a control agent could be seen as such an
assessment. The selection of effective control agents cannot be concluded in
isolation from the effects of land and grazing management (Hatcher and Melander 2003);
synergies with other control methods (Buckley et al. 2004); plant
competition (for example, Fowler and Griffin 1995, Davies et al. 2005); and more
complex tritrophic relationships (Hatcher 1995).